Discover the top five US cities with the worst air pollution and why topographical features make the West Coast so prone
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131 million. That’s the number of Americans – more than one in three – who live in areas with unhealthy levels of air pollution, according to the latest State of the Air Report by the American Lung Association.
At every stage of life, air pollutants have a profound threat upon human health, leading to premature deaths, worsening lung and heart diseases as well as complications during birth.
But where exactly are the worst-hit cities in the country? Here are the top five worst places for air pollution – in reverse order – according to year-round particle pollution:
5) San Jose-San Francisco-Oakland, California
At number five, the metropolitan area’s three major cities – San Jose, San Francisco and Oakland – are all falling short of the national requirements set on air pollution.
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A key factor leading to pollution in the area, along with other cities in California, is the state’s topography. Large mountain ranges – like the Sierra Nevada and the Coast Ranges – surround the central valleys to create a ‘bowl’-like shape. Then, temperature inversions – where air closer to the ground is trapped under a layer of warmer air – acts as a ‘lid’ to the bowl, meaning pollutants sit at levels close to the ground, impacting human and wildlife health along with ecosystem viability.
So significant is the impact of pollution that a Spare The Air programme has been established to combat pollution in the San Francisco Bay Area.
Declared when ground-level ozone is predicted to exceed healthy levels – or when air quality index is more than 100 – residents are asked via television, radio – as well as an app – to reduce driving, gas-powered gardening equipment and stop other polluting activities like painting.
Year round, when a Spare The Air alert is issued, it is illegal for wood to be burned using fireplaces, wood stoves or outdoor fire pits. The small particulates from wood burning can cause serious health issues and is the largest source of air pollution in the Bay Area.
Despite these rules being some of the most stringent in the nation, California still lags behind other US states in its air quality. This has lead state officials to ask the federal government for support, since the source of major sources of pollutants – airplanes, trains and ships – are beyond the jurisdiction of one state alone to control.
4) Eugene-Springfield, Oregon
Located in Oregon, the metropolitan area of Eugene-Springfield faces poor air quality from the main factors of car emissions and wood smoke.
The city of Eugene is at the southern end of the Willamette Valley – far from winds coming in from the Columbia River – meaning it is more susceptible to temperature inversions, and consequently worse air quality. This is because these inversions trap low-lying air closer to the city’s ground – containing harmful particulates and soot – and stops it circulating away. All of this unclean, trapped air with nowhere to go is then inhaled by residents.
To tackle worsening air quality in cities across the state, the Oregon Department of Land Conservation and Development has created a series of rules to be adopted by participating cities in the state.
One rule stipulated that cities including Springfield made at least one or more ‘Climate-Friendly Areas’ – zones which seek to reduce air pollution using methods like implementing high-quality public transport, pedestrian and bicycle routes, or creating urban mixed-use centres with services nearby, so people can complete everyday tasks without relying on a car.
3) Fresno-Madera-Hanford, California
The metropolitan area of Fresno-Madera-Hanford in the San Joaquin Valley of California has one of the worst year-round particle pollutant scores of all states recorded in the 2024 State Of The Air report. Asthma is common in the region, which also had the second-highest average of 54.8 days of high particle pollution per year out of all states recorded in the report.
In California, air pollution can get worse during winter, as the strong bay breeze that moves pollutants during summer months ceases. In these colder months, furnaces and heat pumps are running more frequently, so pollutants build up at even greater speeds with no way for them to be flushed away, according to climatologist at San José State University Eugene Cordero.
In Fresno, the largest city in the metropolitan area, these factors are also compounded by heavy traffic on motorways – especially from diesel trucks, the presence of several industrial facilities and dust from agriculture processing facilities, leading to overall poor air quality.
2) Visalia, California
More than 9 in 10 Californians live in a community with a failing grade for unhealthy ozone pollution, particle pollution days and/or annual particle pollution levels – and the city of Visalia is no exception.
Ranking second, its levels of fine particulate pollution are nearly twice as high as new regulations allow, according to 2020–2022 data from the US Environmental Protection Agency.
The city’s poor air quality is often associated with the Valley’s intensive agricultural uses and two major motorways that run nearby. Smoke from fires, as well as heavy duty trucking emissions, also play a part in Visalia’s levels of pollutants.
1) Bakersfield, California
Located in the major agricultural area of San Joaquin Valley, the city of Bakersfield in California takes the top spot for the city most polluted by short-term and year-round particle pollution.
School officials in the city use flags to denote the air quality on a particular day: green for good, and red as unhealthy for all groups. So significant is the air pollution in Bakersfield that are purple flag was even added to be used on days that are worse than red.
It faces the same topographical factors as other Californian cities on this list – with temperature inversions and the ‘bowl’ shape of land combining together to trap pollutants – as well as experiencing a high level of emissions from nearby rail freight, high volumes of road traffic, intensive agriculture and many other industries.
Around 8 per cent of the city’s residents – or 70,000 people – are reported to have asthma out of the wider metro’s 875,000-strong population.
